1. Field of the Invention
The present invention relates to a suction strainer to remove entrained solids from the cooling liquid in a nuclear reactor, and more particularly, to a suction strainer of modular construction with a contoured surface configuration that reduces head loss across the strainer in the presence of liquids with entrained debris.
2. Description of Related Art
A nuclear power plant typically includes an emergency core cooling system (ECCS) that circulates large quantities of cooling water to critical reactor areas in the event of accidents. A boiling water reactor (BWR) commonly draws water from one or more reservoirs, known as suppression pools, in the event of a loss of coolant accident (LOCA). Water is pumped from the suppression pool to the reactor core and then circulated back to the suppression pool. A LOCA can involve failure of reactor components that introduce large quantities of solid matter into the cooling water, which entrains the solids and carries them back to the suppression pool. For example, if a LOCA results from the rupture of a high pressure pipe, quantities of thermal insulation, concrete, paint chips and other debris can be entrained in the cooling water. A pressurized water reactor (PWR) after a LOCA typically draws cooling water from a reactor water storage tank (RWST), and then after a signal, shuts off the flow from the RWST and recirculates this water through the reactor. That is, a PWR has a containment area that is dry until it is flooded by the occurrence of an accident, and the ECCS uses pump connected to a sump in the containment area to circulate the water through the reactor. Nevertheless, the water that is pumped in the event of a PWR accident will also usually contain entrained solids that typically includes insulation, paint chips, and particulates. In other words, in both types of reactor, cooling water is drawn from a reservoir and pumped to the reactor core, and entrained solids can impair cooling and damage the ECCS pumps if permitted to circulate with the water.
As a result, strainers are typically placed in the coolant flow path upstream of the pumps, usually by immersing them in the cooling water reservoir. It is critical that these strainers be able to remove unacceptably large solids without unduly retarding the flow of coolant; in other words, the pressure (head) loss across the strainer must be kept to a minimum. Strainers are commonly mounted to pipes that are part of the ECCS and that extend into the suppression pool (BWR) or sump (PWR), and the ECCS pumps draw water through the strainers and introduce it to the reactor core. There has been considerable effort expended on the design of strainers to decrease head loss across the strainer for the design coolant flow. These strainers typically include a series of stacked perforated hollow discs (BWRs) or flat perforated plates (PWRs) and a central core through which water is drawn by the ECCS pump. The perforated discs prevent debris larger than a given size from passing the strainer perforations and reaching the pumps. An example of a particularly effective strainer design is the present inventors' U.S. Pat. No. 5,759,399, which is assigned to the same assignee as the present invention and is incorporated herein by reference (as discussed in detail below).
Large amounts of fibrous material can enter the circulating coolant water in the event of a reactor accident. This fibrous material, which originates with reactor pipe or component insulation that is damaged and enters the ECCS coolant stream in the event of a LOCA, accumulates on the strainer surfaces and captures fine particulate matter in the flow. The resulting fibrous debris bed on the strainer surfaces can quickly block the flow through the strainer, even though the trapped particulates may be small enough to pass through the strainer perforations. Heretofore, this flow blockage effect has been addressed by making the strainer larger, the goal being to distribute the trapped debris over more area, reduce the velocity through the debris bed, and thus reduce the head loss across the strainer as a whole. This is, however, an imperfect solution, both because the available space in a reactor for suction strainers is usually limited, and because larger strainers are more costly. Accordingly, it is possible that the expected debris load after a LOCA can dictate strainers that are too large for the space allotted for them in the containment area. Moreover, larger strainers are more difficult to work with and thus more costly to install.
Prior art ECCS strainers have also been constructed in ways that can make them somewhat expensive to fabricate. As a result of all of the above factors, it has proven difficult to reduce the costs of strainers for a nuclear power plant ECCS to any meaningful degree and to provide strainers that fit within the space constraints.